Everything in motion High precision MEMS inertial sensors and systems

Silicon Sensing specialises in the development and manufacture of precise, reliable and affordable MEMS gyroscopes, accelerometers and inertial systems. Established in 1999, but with a heritage in gyroscope technology dating back 100 years, Silicon Sensing has supplied over 20 million MEMS inertial sensors to all commercial and industrial markets. Our mission statement ...

"To be the leading provider of affordable, high performance, high integrity MEMS inertial products and foundry services."

To support our existing business and enable future growth, we are seeking a MEMS Engineer to join our multi-disciplinary technology development team to design and develop next generation MEMS inertial sensors.

The CRM100 gyro can provide a very similar performance to the CRG20 when it is being used in an analogue configuration. There have been improvements made since the parts were first released and while the data sheet figures have remained the same the typical performance has improved (8⁰/hr bias instability). By using two CRM100 and summing the output with a gain of 0.5 then the errors are generally reduced by a factor of √2 (ie 8/√2 = 5.7⁰/hr – very close to the performance of the CRG20) . These error can be reduced further by mounting one CRM100 ‘upside down’ (on the reverse of the pcb) and summing the difference of the outputs, reducing common mode errors.

Alternatively, the CRS43 (which uses the CRM100 internally) has a 5V input – the same as the CRG20.

Due to the very low current used by PinPoint®, interference through the power supply is very unlikely. However, we do recommend using “star points” and good tracking policies to give optimal isolation.

The basic inertial data output from DMU30 is angular rate (deg/s) and acceleration (g). Delta Theta and Delta Velocity outputs can be considered as a re-scaling of these basic data outputs. Provision and use of these values is an industry standard expected and used by many of our customers.

Delta Theta is in degrees and represents the degrees rotated over the sampling time of 5.0ms (or 1/200th of a second). For example, if the IMU axis is rotating at 200deg/s, Delta Theta will be 1 deg.

Similarly, Delta Velocity is a re-scaling from g to m/s, representing the change in velocity change over the same 5ms. The output in m/s is equal to the acceleration in g multiplied by 0.04903325 (divide by 200 and multiply by 9.80665, the latter being a universal measure of g).

Sampling and averaging of the gyro rate data, guided by Allan Variance analysis, is key to getting the best out of CRS39 and to allow the earth rate signal to be drawn out of the noise.

Two sampling schemes that we would recommend are:

(a) In our test chambers we use a16 bit (successive approximation register ADC) National Instruments card. We sample the CRS39-03 fully differentially (Rate and Ref), at 10KHz, without any anti-aliasing filters. We then average every set of 10 samples to produce data at 1 KHz. This data is then analysed for AV and Noise.

(b) In our IMUs, we use 24 bit sigma delta ADCs, outputting sampled data at 10 kHz. Actual sampling at the sensor end is around 192KHz. Again the CRS39-03 is sampled differentially (Rate and Ref). We average every 10 samples to produce a data set at 1KHz.

Analysis of the resulting data using Allan Variance techniques will determine the optimum averaging time. However, the optimum averaging time may be longer than the user can accept and we have typically used 15s averaging for each compass point (90deg separation) measurement.

Minimising temperature variation over measurement cycle will improve accuracy, either by thermal shielding or provision of thermal mass. Improvements may also be found by fully enclosing the gyro in a metal enclosure, minimising any 'metal detector' or field effects.

Temperature compensation is recommended - linear or third order may be required depending on the actual conditions seen by the gyro - temperature range, rate of change of temperature, as well as actual rates applied and temperature.

Averaging measurements taken at index positions of 180deg to each other can help by draw out the real bias of the gyro by removing earth rate. Temperature fitting is also possible by comparing changes between measurements at the same index position against bias and temperature.

Silicon Sensing Systems Ltd is one half of a joint venture based in Plymouth, in the South West of England, UK. It is a gyroscope and inertial systems engineering development company.

Jointly owned by UTC Aerospace Systems, who are based on the same site in Plymouth, and Sumitomo Precision Products, the company was formed in 1999. The two companies each bring an equal strength to the joint venture.